Hybrid vehicles are being developed or manufactured to an increasing extent by the automobile industry. The hybrid vehicles, as plug-in hybrid vehicles, have an internal combustion engine and an electric motor. The hybrid vehicle may therefore be driven by the internal combustion engine using a fossil fuel and/or by the electric motor using electrical power from batteries. Charging of the batteries as traction batteries for driving the plug-in hybrid vehicle takes place during idle time of the hybrid vehicle on an external power network, e.g., having a voltage of 220 V. For this purpose, the hybrid vehicle is to be connected to this external power network with the aid of a power cable and a plug connection. A charging device is required for charging the batteries of the hybrid vehicle, which converts the AC voltage of the external power network into DC voltage and transforms the voltage to an extent required for charging the batteries. Such a charging device installed in the hybrid vehicle discharges waste heat during the charging of the batteries of the hybrid vehicle, which is in the range of several hundred watts up to several kilowatts.
German Patent Application No. DE 10 2008 037 820 A1 describes a method for heating the engine oil of a diesel engine of a hybrid electric vehicle. The hybrid electric vehicle is provided with an electric drive system having a cooling circuit and a heat exchanger. The waste heat of the electric drive system is relayed to the engine oil of the diesel engine with the aid of the cooling circuit and the heat exchanger. The electric drive system is therefore cooled and the engine oil of the diesel engine is heated with the aid of the cooling circuit.